Aggregating several columns to single column - sql

Question

I have a table that looks like
id cat data
--------------------
1 1 foo
2 1 bar
3 1 baz
4 2 some
5 2 random
6 3 Data 1
7 2 data
8 3 Data 2
9 3 Data 3
And I want the last 3 ids and data of each category in a single row like
cat id1 data1 id2 data2 id3 data3
-----------------------------------------------------
1 1 foo 2 bar 3 baz
2 4 some 5 random 7 data
3 6 Data 1 8 Data 2 9 Data 3
I already tried the following:
Get the data with the highest id for each cat:
SELECT id, data FROM tbl t1 WHERE EXISTS (
SELECT 1 FROM tbl t2 WHERE t1.cat = t2.cat
GROUP BY t2.cat HAVING MAX(t2.id) = t1.id
)
Get the data with the 2nd highest ids for each cat:
SELECT id, data FROM tbl t1 WHERE EXISTS (
SELECT 1 FROM tbl t2 WHERE t1.cat = t2.cat AND NOT EXISTS (
-- Not the highest value
SELECT 1 FROM tbl t3 WHERE t1.cat = t3.cat GROUP BY t3.cat
HAVING MAX(t3.id) = t2.id
) GROUP BY t2.cat HAVING MAX(t2.id) = t1.id
)
Get the data with the 3rd highest id for each cat:
SELECT id, data FROM tbl t1 WHERE EXISTS (
SELECT 1 FROM tbl t2 WHERE t1.cat = t2.cat AND NOT EXISTS (
-- id is not 2nd highest
SELECT 1 FROM tbl t3 WHERE t1.cat = t3.cat AND NOT EXISTS (
-- id is not the highest
SELECT 1 FROM tbl t4 WHERE t1.cat = t4.cat GROUP BY t4.cat
HAVING MAX(t4.id) = t3.id
) GROUP BY t3.cat HAVING MAX(t3.id) = t2.id
) AND NOT EXIST (
-- not the highest id
SELECT 1 FROM tbl t5 WHERE t1.cat = t5.cat GROUP BY t5.cat
HAVING MAX(t5.id) = t2.id
) GROUP BY t2.cat HAVING MAX(t2.id) = t1.id
)
And now, joining the entire thing. But I believe that there exists a better solution. What is it?
PS: I have to do it with Informix

Answer 1

Not my answer, a coworker of mine came up with this:
create temp table t(
id smallint,
cat smallint,
data char(10)
) with no log;
insert into t values (1, 1, "foo");
insert into t values (2, 1, "bar");
insert into t values (3, 1, "baz");
insert into t values (4, 2, "some");
insert into t values (5, 2, "random");
insert into t values (6, 3, "Data 1");
insert into t values (7, 2, "data");
insert into t values (8, 3, "Data 2");
insert into t values (9, 3, "Data 3");
insert into t values (10, 4, "some");
insert into t values (11, 4, "more");
insert into t values (12, 4, "random");
insert into t values (13, 4, "data");
insert into t values (14, 4, "for");
insert into t values (15, 4, "testing");
insert into t values (16, 5, "one");
select
cat,
max(case when cnt = 3 then id end) as id1,
max(case when cnt = 2 then id end) as id2,
max(case when cnt = 1 then id end) as id3,
max(case when cnt = 3 then data end) as data1,
max(case when cnt = 2 then data end) as data2,
max(case when cnt = 1 then data end) as data3
from
(
select
a.cat,
a.id,
a.data,
count(*) as cnt
from
t a,
t b
where
a.cat = b.cat and
a.id <= b.id
group by
a.id,
a.cat,
a.data
having
count(*) <= 3
)
group by
1
order by
1;
cat id1 id2 id3 data1 data2 data3
1 1 2 3 foo bar baz
2 4 5 7 some random data
3 6 8 9 Data 1 Data 2 Data 3
4 13 14 15 data for testing
5 16 one

Answer 2

If you using Informix 11.50 or above , there is an option where isn't perfect, but maybe can help. Check the select at end bellow.
They will return a multiset datatype with char() data type... where probably will create a difficult to read it, depending of the program language are you using.
Thanks to Fernando Nunes who suggest this SQL into IIUG forum
At this moment I don't see other alternative besides complex SQLs.
drop table teste;
create temp table teste ( id smallint, cat smallint, data char(10));
insert into teste values ( 1, 1, 'foo ' );
insert into teste values ( 2, 1, 'bar ' );
insert into teste values ( 3, 1, 'baz ' );
insert into teste values ( 4, 2, 'some ' );
insert into teste values ( 5, 2, 'random ' );
insert into teste values ( 6, 3, 'Data 1 ' );
insert into teste values ( 7, 2, 'data ' );
insert into teste values ( 8, 3, 'Data 2 ' );
insert into teste values ( 9, 3, 'Data 3 ' );
insert into teste values ( 10, 3, 'Data 4 ' );
select * from teste;
select ms.*
from
(
SELECT MULTISET( SELECT ITEM t.id || ',' || t.cat || ',' || t.data m1 FROM
teste t WHERE t.cat = tout.cat) FROM (SELECT unique cat from teste) tout
) msdrop table teste;
will return :
expression MULTISET{'1,1,foo ','2,1,bar ','3,1,baz '}
expression MULTISET{'4,2,some ','5,2,random ','7,2,data '}
expression MULTISET{'6,3,Data 1 ','8,3,Data 2 ','9,3,Data 3 '}